The atmosphere is an elastic fluid consisting of a mixture (not a
compound) of oxygen and nitrogen, in the proportions of about twenty-one
of the first to seventy-nine parts of the last named. It also contains a
small quantity of carbonic acid gas, and a yet smaller quantity of
ammonia; and water in the form of invisible vapor is always present in
it, though the quantity is subject to great variations. All these
substances move freely among each other, and are continually changing
places: the oxygen being ever ready to perform the office assigned to it
of sustaining life and combustion; the carbonic acid to promote the
growth of vegetation; the nitrogen to perfect the fruits of the earth,
and the vapor to descend to the thirsty ground, in the form of showers
and dew.
The atmosphere is elastic, and therefore capable of expansion and
compression; and is also a ponderable body. The consequence of these
properties is, that it is much lighter and thinner in the upper regions
than nearer the earth’s surface; for at the sea-level its whole weight
presses on its lower strata and gives it greater density. Ascending from
the earth’s surface it becomes gradually lighter and thinner, and at
great elevations is so rarefied as to be unsusceptible of sustaining
life.